Steel has to be produced with the materials properties requested by the customer. Important materials properties of steel include, for example, the tensile strength, the yield strength and the elongation at break. These materials properties play an important role if the steel is intended for deep-drawing, for example for the production of automobile body panels or drinks cans.
The materials properties of the steel result from its microstructure, which is turn influenced and defined by the production guidelines for the production process.
During production, it is necessary to ensure that the chemical composition of the steel corresponds to the respective setpoint values. These setpoint values correspond to quantitative fractions which can be checked by a chemical laboratory analysis.
To produce a steel which complies with the stipulations, it is also necessary to maintain defined operating parameters. These operating parameters include setpoint values for the melting, casting, heating, rolling and cooling phase. They may, for example, be temperatures, pressures, velocities or changes of these parameters over the course of time.
A setpoint value for a defined quantitative fraction for an element or a setpoint value for an operating parameter always comprises a target value and a lower limit value and an upper limit value, between which the tolerance range lies.
In steelworks or rolling mills, it is often difficult to define suitable setpoint values for production in such a way that the desired materials properties in the end product are produced under production conditions that are as low-cost as possible and involve the minimum possible risk. The demands of the customer, who for example orders a standardized steel grade for a specific application area, have to be implemented taking into the production processes used in the steelworks. These production processes include the works standards for the steel grade and also the target values and tolerance ranges for the individual production steps.
In practice, however, the actual values from production always deviate from the setpoint values for the production process. The deviations in the actual values in a production step have to be counteracted in the subsequent steps in such a way that the required materials properties, for example the minimum value for the tensile strain, are achieved. If this value can no longer be achieved, the corresponding steel material (the melt, slab or coil) can no longer be used for the intended order. In this case, the processing of this part of the order has to be restarted at the first production step, which leads to financial losses and possibly also to delivery delays.
The defining of suitable setpoint values for production is usually carried out by a planning department as part of technical order planning. The operator makes use of various technical tables, which are stored in a data processing installation, to convert the customer's order into a suitable production process for the plant, this production process being linked to defined setpoint values.
For the steel, the quantitative fractions of the individual elements are defined and can be checked by a chemical laboratory analysis. This set of setpoint values is also referred to as the steel grade. The setpoint values for the operating parameters of the individual production steps are also defined. In the steelworks, the melt is produced and the slabs are cast from this melt. In the furnace, the slabs are heated for the rolling operation. In the hot-rolling mill, the slabs are rolled to form coils, and if appropriate further rolling is carried out in a cold-rolling mill, and/or qualitative heat treatment of the coils and if appropriate division into smaller formats are carried out.
The situation often arises whereby a certain order can be produced by a plurality of alternative sets of setpoint values which result from the tables. To achieve the desired properties, the operator has to decide on one of these possible sets of setpoint values, since the production order to the production department only permits one set of setpoint values. The operator decides on a defined set of setpoint values based on his production experience. The entire know-how of the company relating to production operations is held within these technical tables, which are looked after by the quality division. New entries and changes in these tables result from long years of production experience and from test productions using sets of setpoint values which have been altered in steps.
In recent years, mathematical methods have been developed for calculating the materials properties of steel. These methods include a physico-metallurgical model of the production process and determine the changes in the microstructure of the steel for each production step and then calculate the materials properties from this information.
Individual production steps can be simulated on the computer in this way. For standard steel grades, it has been ascertained that the materials properties which are calculated are well matched to the values which are actually measured.
EP 0 901 016 A2 has disclosed a mathematical method for determining properties of a steel. Typical variables, such as yield strength, tensile strength, elongation at break, hardness, etc. are supposed to be determined by means of a neural network.
WO 99/24182 has disclosed a method for controlling a metallurgical installation for the production of steel or aluminum, in which steel is produced with defined materials properties that are dependent on the microstructure, the materials properties being dependent on operating parameters with which the installation is operated. The operating parameters are supposed to be defined by means of a microstructure optimizer as a function of the desired materials properties of the steel. The microstructure optimizer is a computation program for microstructure optimization which makes use of neural networks.
However, a drawback is that the production planning is often unable to produce a successful production program, i.e. an appropriate sequence, since it is restricted to one set of setpoint values per production order. One such example is melt planning, which has to combine a large number of production orders with different steel grades and dimensions in the form of melts and sequences, i.e. technically compatible sequences of melts. Only orders of one steel grade can be processed in one melt, and only melts with compatible steel grades can follow one another in one sequence. The stock of orders often comprises a large number of qualitatively similar orders of small quantities which, however, have different steel grades. This situation often leads to poor production programs, since the number of possible melts in the program is small and the individual melts often have to be filled with what are known as store orders, since there is an absence of customer orders for suitable qualities.
Another drawback is that the quality department has to spend considerable money and time on optimizing sets of setpoint values with regard to the demands on the materials properties and with regard to production costs. The large number of production tests required, with modified sets of setpoint values, and the testing of the materials properties using laboratory specimens which in each case ensues, lead to high levels of outlay. Therefore, many sets of setpoint values are compromised in terms of reliability with regard to the desired target values and tolerance ranges since they have had to be derived from an inadequate number of production tests. Larger tolerance ranges and more cost-effective target values for certain materials properties can only be implemented after a steel grade has been produced for a number of years.
In principle, the control systems of the individual production units attempt to maintain their setpoint values for the output variables even if the input values, i.e. the actual values from the preceding step, deviate. If a slab arrives at the rolling train from the furnace at a temperature which is too cold, the control system of the rolling train nevertheless attempts to reach its setpoint value for the output temperature of the rolled strip by suitably changing the control variables of the control system. This only works for setpoint values which can still be influenced, for example incorrect temperatures or dimensions which can be changed to a certain extent. However, it is not possible for a steel with an incorrect chemical analysis, i.e. with incorrect quantitative fractions of the alloying elements, to be altered in the subsequent rolling mill.
If the deviations cannot be tolerated, for example because individual elements in the chemical analysis infringe the limit values, the current intermediate product is given up for this order. Production planning then has to produce further material with a high priority, but this leads to time delays. The current intermediate material is either switched to other suitable orders, i.e. reassigned, or is if necessary switched to store orders without any associated customer request, or is removed from production and placed in an intermediate store. The final option is for the material to be designated scrap and return to the resource circuit.